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I am an undergraduate electrical engineering student, and as a final project for one of my classes I made a speaker system which could play music connected through a 3mm cable. The system was powered by the power supply units we have in the lab.

The project is over now, but i still want to be able to use my speaker out of the lab. It currently needs to be hooked up to 12V+, 12V- and ground in order to function. Is there any way I could use batteries, perhaps 12V batteries, as the power source to make this mobile?

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2 Answers

Just connect the + of one battery to the - of the other. The point where the batteries connect is ground, the + terminal is +12V and the - terminal is -12V.

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Of course, this is assuming the batteries are really 12V and not something more like 13.4V.

In that case you may be wanting to add a pair of regulators (a positive one and a negative one) or some Zener diodes to cap it at exactly +/-12V if your device can't handle more than 12V (unlikely).

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If the batteries are for example 13.4, and the circuitry you're powering can take more than 12, say it can take 15, then consider just running it at 13.4, especially if it draws a lot of current. If the batteries have slightly different voltages, and you're playing with op-amps, consider a virtual ground instead of taking the mid-point of the batteries as ground. Still, +1 –  JustJeff Dec 9 '11 at 11:39
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If your system will tolerate a range of voltages rather than exactly 12v, then the answer would be yes.

You would need two batteries, one with it's negative terminal connected to ground to provide the "+12v" supply and the other with it's positive terminal connected to ground to provide the "-12v" supply.

It would be a good idea to place fuses right at the battery terminals to protect against accidental short circuits and other faults.

Also, liquid-electrolyte lead-acid batteries require a degree of care in application and may not be the best choice outside of vehicle/marine applications. NiMH cells or sealed gel electrolyte batteries are generally preferable for smaller systems, especially if used indoors.

Finally note that this is only straightforward if your amplifier is only connected to the batteries when they are not being charged. If you remain connected while they are charging (particularly off an engine) additional protection circuitry will be required.

An alternative course of action would be to use one battery and a dual output DC/DC converter. This would also get you a more consistent output voltage. Or you could re-design the circuit to operate from a single supply, using for example a car-audio audio power amp chip.

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